How do you sink?

[SeaHound]

In PADI Peak performance Buoyancy chapter they teach you that proper weighting means that with a totally deflated BCD, you should be in floating at the surface eye level!

How the hell do you sink then? You cant further deflate the BC cuz there is nothing in it. Furthermore what is the purspose of a BC jacket if you can float with a deflated one.

 

[jbd]

In PADI Peak performance Buoyancy chapter they teach you that proper weighting means that with a totally deflated BCD, you should be in floating at the surface eye level!

How the hell do you sink then? You cant further deflate the BC cuz there is nothing in it. Furthermore what is the purspose of a BC jacket if you can float with a deflated one.

When properly weighted all you have to do is relax and exhale.

The BC in BC jacket stands for "bouyancy control". The purpose of this device is to change the volume (therefore density) of the diver to compensate for the effects of increasing pressure as you descend. Read up on Boyles Law and Archimedes principle for a better understanding.

 

[MikeFerrara]

In PADI Peak performance Buoyancy chapter they teach you that proper weighting means that with a totally deflated BCD, you should be in floating at the surface eye level!

...while holding a normal breath. Did you get to that part yet?

Also that's a test to see if you close to neutral but you still need to account for the fact that your tank will be 6 pounds lighter when it's empty so it's either done with a near empty tank or you add a few pounds after you find out what it takes to get you neutral.

How the hell do you sink then? You cant further deflate the BC cuz there is nothing in it. Furthermore what is the purspose of a BC jacket if you can float with a deflated one.

As I stated above you'll be a few pounds heavy with a full tank. When you are pretty neutral with an empty bc and holding a normal breath you just exhale.

The bc is used to compensate for changes in buoyancy during the dive due to expansion/compression of exposure protection and the tank getting lighter as it empties. It's not a life preserver and if you had a exposure suit of constant buoyancy and a tank that somehow didn't get lighter during the dive there wouldn't be a need for one at all.

 

[Marek K]

Yeah, like Mike said... There are four elements to this proper-weight check. You mentioned only two of them... 1) BC deflated; and 2) floating at eye level.

The other two necessary elements are 3) holding a normal breath; and 4) doing this check at the end of your dive when your tank is almost empty and therefore lighter.

Weighted and checked that way, you should theoretically have no trouble descending at the beginning of your dive with a heavier tank and having exhaled completely.

Of course, the dilemma is, this check is only good for dives after the one you're on. You can't do it as neatly with a full tank at the beginning of a dive, when it would be most useful. If you did, and didn't add a bit of weight to compensate for the tank air you'll be using up later, you might find that you were dangerously under-weighted at the end of the dive -- when you're ascending, and when bouyancy is probably most critical.

Also don't forget that stuff (air actually; in your wetsuit neoprene and remnants in your BC) will start to compress once you start descending, and you'll quickly lose buoyancy that way as you go deeper.

--Marek

 

[MXGratefulDiver]

In PADI Peak performance Buoyancy chapter they teach you that proper weighting means that with a totally deflated BCD, you should be in floating at the surface eye level!

How the hell do you sink then? You cant further deflate the BC cuz there is nothing in it. Furthermore what is the purspose of a BC jacket if you can float with a deflated one.

Because you will be floating at eye level while holding a normal inhale's worth of breath in your lungs.

Keep in mind that when you go underwater, you take two BCD's with you ... the one on your back and the one that you were born with (your lungs).

The external one holds more air than the internal one, so you can think of your BCD as "coarse adjust" and your lungs as "fine adjust".

Your lungs will typically provide 6-8 lbs of buoyancy difference between a full inhale and a full exhale. You only need one or two lbs of negative bouyancy to sink. So with air in your lungs, you should only be carrying enough weight to provide a slight amount of negative buoyancy when you completely exhale. And, as Mike said, you will need to add a few extra lbs to adjust for the fact that your cylinder will become more bouyant as you use the gas inside of it.

Ideally, the weight check you describe should be done with only 500 psi in your cylinder, since that's typically your target for reserve at the end of the dive.

The purpose of the BCD is two-fold ... to float you comfortably during surface swims, and to compensate for buoyancy changes as you descend and ascend. Remember that as you go deeper, if you change nothing at all, you will become more negatively buoyant due to pressure changes. As you ascend, changing nothing at all, you will become more positively buoyant. Your BCD gives you the ability to compensate (the C in BCD) for buoyancy changes due to pressure ... you add short bursts of air as you go deeper, and release short bursts of air as you go shallower.

This all should have been covered in your OW class ... if it doesn't make sense, I would recommend that you sign up for a performance buoyancy class. The concepts are fundamental to everything you will do as a diver, and you'll have a lot more fun in the water if you have a solid understanding not only of what's going on, but why ...

... Bob (Grateful Diver)

 

[SeaHound]

Thanks for clearifying guys. The mistake that I was making was doing the check BEFORE the dive instead of after. Thats why it wasnt working.

 

[SubMariner]

Thanks for clearifying guys. The mistake that I was making was doing the check BEFORE the dive instead of after. Thats why it wasnt working.

Actually, you CAN do the check before you dive. You just add 3-5 pounds to compensate for the positive buoyancy of a AL cylinder with 1000 psi or less in it.

 

[springdiver]

...while holding a normal breath. Did you get to that part yet?

Just remember when doing yoga or anything with breathing exercises (even diving) - breath in and breath out Important to do both things! The above explanations seem to be sufficient.

 

[DBailey]

If you are diving in a new environment (fresh instead of salt or vice versa) or with different gear (thicker wetsuit, etc.), then do the weight check at the start of the dive with a full tank. Once you get to the "empty BC, eye level, normal breath" point add the 3-5 lbs for the air that you will use.

This will get you close enough for your first dive. After the first dive, just do more accurate weight check with 500 psi in the tank. Then record everything in your log book for future reference.

 

[Randy43068]

Just dump air from the BC and exhale. I have to tip my left shoulder higher than the right so the air in my BC goes to the highest place so it can vent off. Then down I go.. I have to give a quick squirt of air after about 5 or 6 feet so I don't drop too fast. My ears can't take it... have to go down pretty slow anymore.

Randy

In PADI Peak performance Buoyancy chapter they teach you that proper weighting means that with a totally deflated BCD, you should be in floating at the surface eye level!

How the hell do you sink then? You cant further deflate the BC cuz there is nothing in it. Furthermore what is the purspose of a BC jacket if you can float with a deflated one.